The sense of taste plays an important role in the life of humans and animals. Mammals are generally thought to distinguish five primary taste qualities: sour, salty, bitter, sweet, and umami (the taste of monosodium glutamate, aspartate and some ribonucleotides). These five modalities reflect the organism's internal physiological needs. For example, salty, sweet and umami tastes enable humans and animals to seek out necessary minerals, energy- or nutrient-rich food while sour and bitter perceptions are crucial for the avoidance of putrefied foods, unripe fruits, potentially harmful plant alkaloids and other toxins. Taste stimuli can also evoke positive hedonic responses and, via the cephalic phase, initiate secretory and metabolic changes in the gut prior to food entering the stomach.
Taste sensations are mediated by specialized epithelial cells, referred to as taste receptor cells. About 50-150 taste receptor cells and supporting cells form a single taste bud. These are embedded within the epithelium of the tongue in morphologically distinct papillae or regions. Taste receptor cells utilize different receptors and signaling pathways to convey the tastes of different modalities. For example, salt taste is apparently mediated by sodium channels while sour taste may be transduced by one of several putative channels, including acid-sensing ion channels, hyperpolarization- and cyclic nucleotide-gated cation channels. In addition, electrophysiological studies suggest that chloride channels are also involved in sour taste transduction. Bitter, sweet and umami tastes are transduced by seven transmembrane receptors coupled with heterotrimeric G proteins. In addition, bitter compounds may permeate taste receptor cells and directly interact with G proteins and ion channels. Activation of receptors and ion channels by taste stimuli leads to taste cell depolarization, alteration of membrane potentials, generation of action potentials, and the triggering of neurotransmitter release onto afferent gustatory nerves, transmitting the gustatory signals to the brain.
The molecular mechanisms underlying taste transduction, especially the peripheral signal coding in the taste buds are not well understood. Molecular and genetic data indicated that subsets of taste receptor cells are responsible for bitter, sweet and umami tastes, respectively. Physiological studies showed that most taste receptor cells can be excited by stimuli representative of two or more different taste modalities. Furthermore, the molecules that are involved in generation of action potentials in taste receptor cells and in transmission of gustatory signals from taste receptor cells to afferent axons remain unknown.
Identification of proteins that respond to changes in taste cell membrane potential will allow novel insights into taste peripheral coding. The activity of some of these potential-sensitive proteins may also be pH sensitive. If so, they could represent an additional transduction process in sour taste. Possessing a more complete understanding of the activity of channel proteins will provide new targets for evaluating taste stimuli and modifiers and developing new flavors.
Currently, the molecular mechanisms underlying the transduction steps in taste, and in sour taste sensation in particular, are not fully understood. Therefore, strategies that seek to discover substances to modify tastes and to develop new flavors is based on incomplete knowledge. As a result, many potentially taste active compounds need to be taken through exhaustive and difficult animal feeding studies, or expensive human psychophysical tests. At present, the incomplete knowledge of sour taste and its interaction with other taste modalities makes rational computational design approaches difficult.